1. Field of the Invention
The present invention relates to a system comprising a synchronous terminal, a data transfer arrangement which comprises a modem and permits transferring data between the modem and the synchronous terminal. The data transfer arrangement also comprises, on one hand, a plurality of terminal-connected accesses including a first access for receiving data from the terminal and a second access for supplying clock signals to the terminal for fixing the rate of the received data, and on the other hand accesses connected via the modem to the transmission line for transmitting the data at a uniform rate.
2. Description of the Related Art
A problem in data transmission systems is to adapt the data rate of the terminals to the data rate provided by a transmission line. With respect to this subject the Japanese document KOKAI 63-120537 may be consulted.
An obvious solution to this type of problem is to utilize an asynchronous operating mode according to which a terminal does not transmit until the data transfer arrangement gives it authorization. This mode is also well adapted to use of a modem with which data compression procedures are carried out, more specifically, those satisfying Recommendation V42A of the CCITT. In effect, the level of compression is variable depending on the nature of the data supplied by the terminal. For a transmission line whose rate is limited to 9600 bits per second, data supplied by the terminal at the rate of 19200 bits/s can thus be transmitted if the compression level is higher than two. However, if the compression level is lower than two, the speed of the terminal must be reduced. In the asynchronous mode influence is exerted on either:
(i) the flow of the data supplied to the modem, check bits X.sub.ON (authorization to transmit) or X.sub.OFF (prohibition to transmit) being included, or PA1 (ii) the junction circuit [106] (which circuit is defined in the CCITT Recommendations), so that it switches to the closed state to give the terminal authorization to transmit and switches to the open state to give the terminal a prohibition to transmit. These measures are not applicable when a synchronous type of terminal is concerned. The characteristics X.sub.ON and X.sub.OFF cannot be interpreted, because the terminal transmits and receives successive frames. Also, the junction circuit [106] then has a different meaning. An open state of this circuit generally causes the terminal to be disconnected.